Present-day launch costs are very high – $2,500 to $15,000 per kilogram from Earth to low Earth orbit (LEO). As a result, launch costs are a large percentage of the cost of all space endeavors. If launch can be made cheaper the total cost of space missions will be reduced. Fortunately, due to the exponential nature of the rocket equation, providing even a small amount of the velocity to LEO by other means has the potential of greatly reducing the cost of getting to orbit.

A space tower is a tower that would reach outer space. To avoid an immediate need for a vehicle launched at orbital velocity to raise its perigee, a tower would have to extend above the edge of space (above the 100 km Kármán line),[22] but a far lower tower height could reduce atmospheric drag losses during ascent. If the tower went all the way to geosynchronous orbit at approximately 36,000 km, or 22,369 miles, objects released at such height could then drift away with minimal power and would be in a circular orbit. The concept of a structure reaching to geosynchronous orbit was first conceived by Konstantin Tsiolkovsky.[23] The original concept envisioned by Tsiolkovsky was a compression structure. Building a compression structure from the ground up proved an unrealistic task as there was no material in existence with enough compressive strength to support its own weight under such conditions.[24] Other ideas use very tall compressive towers to reduce the demands on launch vehicles. The vehicle is "elevated" up the tower, which may extend above the atmosphere and is launched from the top. Such a tall tower to access near-space altitudes of 20 km (12 mi) has been proposed by various researchers.[25][26]

The height is limited by materials,[27] with higher structures possible if the structure tapers (i.e. the upper parts are narrower than the bottom), but with current construction techniques, cost increases exponentially with construction height. Buckling may be a failure mode before exceeding a material's nominal compressive yield strength (though designs such as with a truss may help compensate), but, aside from that and aside from design against weather, the theoretical scale height of a structure is the allowable load of its material divided by the product of density and local gravitational acceleration, where needed material cross-section increases by a factor of e (2.718...) over each scale height.[28]

For common plain carbon steel under a typical allowable stress limit, its scale height is ≈ 1.635 kilometer.[citation needed] A 4.9-kilometer-high tower (3 × its scale height) of such would accordingly mass at least 20 times the weight supported at its top (as e3≈ 20). In contrast, an example of a more expensive high-performance aerospace material, Amoco T300/ERL1906 carbon composite, has a scale height of 54 kilometers at a safety factor of 2, though construction challenges, including wind loading, would apply. Earth's atmosphere has approximately 50% of its mass under 6 kilometers elevation, 90% below 16 kilometers, and 99% below 30 kilometers of altitude.[28][29]

Tensile structures for non-rocket spacelaunch are proposals to use long, very strong cables (known as tethers) to lift a payload into space. Tethers can also be used for changing orbit once in space.

Orbital tethers can be tidally locked (skyhook) or rotating (rotovators). They can be designed (in theory) to pick up the payload when the payload is stationary or when the payload is hypersonic (has a high but not orbital velocity).[citation needed]

Endo-atmospheric tethers can be used to transfer kinetics (energy and momentum) between large conventional aircraft (subsonic or low supersonic) or other motive force and smaller aerodynamic vehicles, propelling them to hypersonic velocities without exotic propulsion systems.[citation needed]

A skyhook is a theoretical class of orbiting tether propulsion intended to lift payloads to high altitudes and speeds.[30][31] Proposals for skyhooks include designs that employ tethers spinning at hypersonic speed for catching high speed payloads or high altitude aircraft and placing them in orbit.[32]

A space elevator would consist of a cable anchored to the Earth's surface, reaching into space.

A space elevator is a proposed type of space transportation system.[33] Its main component is a ribbon-like cable (also called a tether) anchored to the surface and extending into space above the level of geosynchronous orbit. As the planet rotates, the centrifugal force at the upper end of the tether counteracts gravity, and keeps the cable taut. Vehicles can then climb the tether and reach orbit without the use of rocket propulsion.

Such a cable could be made out of any material able to support itself under tension by tapering the cable's diameter sufficiently quickly as it approached the Earth's surface. On Earth, with its relatively strong gravity, current materials are not sufficiently strong and light. With conventional materials, the taper ratio would need to be very large, increasing the total launch mass to a fiscally infeasible degree. However, carbon nanotube or boron nitride nanotube based materials have been proposed as the tensile element in the tether design. Their measured strengths are high compared to their linear densities. They hold promise as materials to make an Earth-based space elevator possible.[34]

Landis and Cafarelli suggested that a tension structure ("space elevator") extending downward from geosynchronous orbit could be combined with the compression structure ("Tsiolkovski tower") extending upward from the surface, forming the combined structure reaching geosynchronous orbit from the surface, and having structural advantages over either one individually.[24]

The space elevator concept is also applicable to other planets and celestial bodies. For locations in the Solar System with weaker gravity than Earth's (such as the Moon or Mars), the strength-to-density requirements aren't as great for tether materials. Currently available materials (such as Kevlar) could serve as the tether material for elevators there.

An endo-atmospheric tether uses the long cable within the atmosphere to provide some or all of the velocity needed to reach orbit. The tether is used to transfer kinetics (energy and momentum) from a massive, slow end (typically a large subsonic or low supersonic aircraft) to a hypersonic end through aerodynamics or centripetal action. The Kinetics Interchange TEther (KITE) Launcher is one proposed endo-atmospheric tether.[15]

A space fountain is a proposed form of space elevator that does not require the structure to be in geosynchronous orbit, and does not rely on tensile strength for support. In contrast to the original space elevator design (a tetheredsatellite), a space fountain is a tremendously tall tower extending up from the ground. Since such a tall tower could not support its own weight using traditional materials, massive pellets are projected upward from the bottom of the tower and redirected back down once they reach the top, so that the force of redirection holds the top of the tower aloft.[citation needed]

An orbital ring is a concept for a space elevator that consists of a ring in low Earth orbit that rotates at slightly above orbital speed, and has fixed tethers hanging down to the ground.[citation needed]

In the 1982 Paul Birch JBIS design[36] of an orbital ring system, a rotating cable is placed in a low Earth orbit, rotating at slightly faster than orbital speed. Not in orbit, but riding on this ring, supported electromagnetically on superconducting magnets, are ring stations that stay in one place above some designated point on Earth. Hanging down from these ring stations are short space elevators made from cables with high tensile-strength-to-mass ratio. Birch claimed that the ring stations, in addition to holding the tether, could accelerate the orbital ring eastwards, causing it to precess around Earth. If it were possible to make the precession rate large enough – once per day, the rate of rotation of the Earth – the ring would be "geostationary" without having to be at the normal geostationary altitude or even in the equatorial plane.[citation needed]

One proposed design is a freestanding tower composed of high strength material (e.g. kevlar) tubular columns inflated with a low density gas mix, and with dynamic stabilization systems including gyroscopes and "pressure balancing".[39] Suggested benefits in contrast to other space elevator designs include avoiding working with the great lengths of structure involved in some other designs, construction from the ground instead of orbit, and functional access to the entire range of altitudes within the design's practical reach. The design presented is "at 5 km altitude and extending to 20 km above sea level", and the authors suggest that "the approach may be further scaled to provide direct access to altitudes above 200 km".

A major difficulty of such a tower is buckling since it is a long slender construction.

With any of these projectile launchers, the launcher gives a high velocity at, or near, ground level. In order to achieve orbit, the projectile must be given enough extra velocity to punch through the atmosphere, unless it includes an additional propulsion system (such as a rocket). Also, the projectile needs either an internal or external means to perform orbital insertion. The designs below fall into three categories, electrically driven, chemically driven, and mechanically driven.

Electrical launch systems include mass drivers, railguns, and coilguns. All of these systems use the concept of a stationary launch track which uses some form of linear electrical motor to accelerate a projectile.

A mass driver is basically a very long and mainly horizontally aligned launch track or tunnel for space launch, curved upwards at the end. The concept was proposed by Arthur C. Clarke in 1950,[40] and was developed in more detail by Gerard K. O'Neill, working with the Space Studies Institute, focusing on the use of a mass driver for launching material from the Moon.

A mass driver uses some sort of repulsion to keep a payload separated from the track or walls. Then it uses a linear motor (an alternating-current motor such as in a coil gun, or a homopolar motor as in a railgun) to accelerate the payload to high speeds. After leaving the launch track, the payload would be at its launch velocity.

StarTram is a proposal to launch vehicles directly to space by accelerating them with a mass driver. Vehicles would float by maglev repulsion between superconductive magnets on the vehicle and the aluminum tunnel walls while they were accelerated by AC magnetic drive from aluminum coils. The power required would probably be provided by superconductive energy storage units distributed along the tunnel. Vehicles could coast up to low or even geosynchronous orbital height; then a small rocket motor burn would be required to circularize the orbit.

Cargo-only Generation 1 systems would accelerate at 10–20 Gs and exit from a mountain top. While not suitable for passengers, they could put cargo into orbit for $40 per kilogram, 100 times cheaper than rockets.

Passenger-capable Generation 2 systems would accelerate for a much longer distance at 2 Gs. The vehicles would enter the atmosphere at an altitude of 20 km from an evacuated tunnel restrained by Kevlar tethers and supported by magnetic repulsion between superconducting cables in the tunnel and on the ground. For both Gen 1–2 systems, the mouth of the tube would be open during vehicle acceleration, with air kept out by magnetohydrodynamic pumping.[16][41][42]

However, even with a "gun barrel" through both the Earth's crust and troposphere, the g-forces required to generate escape velocity would still be more than what a human tolerates. Therefore, the space gun would be restricted to freight and ruggedized satellites. Also, the projectile needs either an internal or external means to stabilize on orbit.

Gun launch concepts do not always use combustion. In pneumatic launch systems, a projectile is accelerated in a long tube by air pressure, produced by ground-based turbines or other means. In a light-gas gun, the pressurant is a gas of light molecular weight, to maximize the speed of sound in the gas.

In the 1990s, John Hunter of Quicklaunch proposed use of a 'Hydrogen Gun' to launch unmanned payloads to orbit for less than the regular launch costs.[43]

A ram accelerator also uses chemical energy like the space gun but it is entirely different in that it relies on a jet-engine-like propulsion cycle utilizing ramjet and/or scramjet combustion processes to accelerate the projectile to extremely high speeds.

It is a long tube filled with a mixture of combustible gases with a frangible diaphragm at either end to contain the gases. The projectile, which is shaped like a ram jet core, is fired by another means (e.g., a space gun, discussed above) supersonically through the first diaphragm into the end of the tube. It then burns the gases as fuel, accelerating down the tube under jet propulsion. Other physics come into play at higher velocities.

A blast wave accelerator is similar to a space gun but it differs in that rings of explosive along the length of the barrel are detonated in sequence to keep the accelerations high. Also, rather than just relying on the pressure behind the projectile, the blast wave accelerator specifically times the explosions to squeeze on a tail cone on the projectile, as one might shoot a pumpkin seed by squeezing the tapered end.

In a slingatron,[19][44]projectiles are accelerated along a rigid tube or track that typically has circular or spiral turns, or combinations of these geometries in two or three dimensions. A projectile is accelerated in the curved tube by propelling the entire tube in a small-amplitude circular motion of constant or increasing frequency without changing the orientation of the tube, i.e. the entire tube gyrates but does not spin. An everyday example of this motion is stirring a beverage by holding the container and moving it in small horizontal circles, causing the contents to spin, without spinning the container itself.

This gyration continually displaces the tube with a component along the direction of the centripetal force acting on the projectile, so that work is continually done on the projectile as it advances through the machine. The centripetal force experienced by the projectile is the accelerating force, and is proportional to the projectile mass.

In air launch, a carrier aircraft carries the space vehicle to high altitude and speed before release. This technique was used on the suborbital X-15 and SpaceshipOne vehicles, and for the Pegasus orbital launch vehicle.

The main disadvantages are that the carrier aircraft tends to be quite large, and separation within the airflow at supersonic speeds has never been demonstrated, thus the boost given is relatively modest.

Early spaceplanes were used to explore hypersonic flight (e.g. X-15).[45]

Some air-breathing engine-based designs (cf X-30) such as aircraft based on scramjets or pulse detonation engines could potentially achieve orbital velocity or go some useful way to doing so; however, these designs still must perform a final rocket burn at their apogee to circularize their trajectory to avoid returning to the atmosphere. Other, reusable turbojet-like designs like Skylon which uses precooled jet engines up to Mach 5.5 before employing rockets to enter orbit appears to have a mass budget that permits a larger payload than pure rockets while achieving it in a single stage.

However, balloons have relatively low payload (although see the Sky Cat project for an example of a heavy-lift balloon intended for use in the lower atmosphere), and this decreases even more with increasing altitude.

The lifting gas could be helium or hydrogen. Helium is not only expensive in large quantities but is also a nonrenewable resource. This makes balloons an expensive launch assist technique. Hydrogen could be used as it has the advantage of being cheaper and lighter than helium, but the disadvantage of also being highly flammable. Rockets launched from balloons, known as " rockoons", have been demonstrated but to date, only for suborbital ("sounding rocket") missions. The size of balloon that would be required to lift an orbital launch vehicle would be extremely large.

One prototype of a balloon launch platform has been made by JP Aerospace as "Project Tandem",[46] although it has not been used as a rocket launch vehicle. A Spanish company, zero2infinity, is officially developing a launcher system called bloostar based on the rockoon concept, expected to be operational by 2018.[47]

Gerard K. O'Neill proposed that by using very large balloons it may be possible to construct a space port in the stratosphere. Rockets could launch from it or a mass driver could accelerate payloads into the orbit.[48] This has the advantage that most (about 90%) of the atmosphere is below the space port.

Artist depiction of an aerial view of a SpaceShaft.

A SpaceShaft is a proposed version of an atmospherically buoyant structure that would serve as a system to lift cargo to near-spacealtitudes, with platforms distributed at several elevations that would provide habitation facilities for long term human operations throughout the mid-atmosphere and near-space altitudes.[49][50][51] For space launch, it would serve as a non-rocket first stage for rockets launched from the top.[50]

Separate technologies may be combined. In 2010, NASA suggested that a future scramjet aircraft might be accelerated to 300 m/s (a solution to the problem of ramjet engines not being startable at zero airflow velocity) by electromagnetic or other sled launch assist, in turn air-launching a second-stage rocket delivering a satellite to orbit.[52]

All forms of projectile launchers are at least partially hybrid systems if launching to low Earth orbit, due to the requirement for orbit circularization, at a minimum entailing several percent of total delta-v to raise perigee (e.g. a tiny rocket burn), or in some concepts much more from a rocket thruster to ease ground accelerator development.[16]

Some technologies can have exponential scaling if used in isolation, making the effect of combinations be of counter-intuitive magnitude. For instance, 270 m/s is under 4% of the velocity of low Earth orbit, but a NASA study estimated that Maglifter sled launch at that velocity could increase the payload of a conventional ELVrocket by 80% when also having the track go up a 3000‑meter mountain.[53]

A proposal to use a hybrid system combining a mass driver for initial lofting followed by additive thrust by a series of ground-based lasers sequenced according to wavelength was proposed by Mashall Savage in the book The Millennial Project as one of the core theses of the book, but the idea has not been pursued to any notable degree. Savage's specific proposals proved to be infeasible on both engineering and political grounds, and while the difficulties could be overcome, the group Savage founded, now called the Living Universe Foundation, has been unable to raise significant funds for research.

Combining multiple technologies would in itself be an increase to complexity and development challenges, but reducing the performance requirements of a given subsystem may allow reduction in its individual complexity or cost. For instance, the number of parts in a liquid-fueled rocket engine may be two orders of magnitude less if pressure-fed rather than pump-fed if its delta-v requirements are limited enough to make the weight penalty of such be a practical option, or a high-velocity ground launcher may be able to use a relatively moderate performance and inexpensive solid fuel or hybrid small motor on its projectile.[55]Assist by non-rocket methods may compensate against the weight penalty of making an orbital rocket reusable. Though suborbital, the first private manned spaceship, SpaceShipOne had reduced rocket performance requirements due to being a combined system with its air launch.[56]

^Hirschfeld, Bob (2002-01-31). "Space Elevator Gets Lift". TechTV. G4 Media, Inc. Archived from the original on 2005-06-08. Retrieved 2007-09-13. The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his "Speculations about Earth and Sky and on Vesta." Citation copied from Space elevator article. As of Oct 2012, the archived page was still good, but the original no longer functioned.

^Hirschfeld, Bob (2002-01-31). "Space Elevator Gets Lift". TechTV. G4 Media, Inc. Archived from the original on 2005-06-08. Retrieved 2007-09-13. The concept was first described in 1895 by Russian author K. E. Tsiolkovsky in his "Speculations about Earth and Sky and on Vesta".

^In his book "Structures or why things don't fall down" (pub [clarification needed] Pelican 1978 – 1984 [clarification needed]), professor J. E. Gordon considers the height of the Tower of Babel. He wrote: brick and stone weigh about 120 LB/ft^3, or 2,000 kg/m^3, and the crushing strength of these materials is approximately 6000 PSI, or 40M N/m^2. Elementary arithmetic shows that a tower with parallel walls could have been built to a height of 7000 feet, or 2 kilometers, before the bricks at the bottom were crushed. However by making the walls taper towards the top they could well have been built to a height where the men of Shinnar would run short of oxygen before the brick walls crushed beneath their own dead weight." [clarification needed]